Conventionally electrically conductive powder used for electrically and thermally conductive pastes is a mixture of large and small, spherical or substantially spherical particles for high-density packing (Nonpatent Document 1). In the fields where high electrical or thermal conductivity is required, electrically conductive powders such as of gold, silver, copper, aluminum, palladium and alloys of these metals are used. The electrical or thermal conductivity is improved by increasing the amount of the electrically conductive powder. (Nonpatent Document 1: FUNTAI KOUGAKU BINRAN (POWDER TECHNOLOGY HANDBOOK), first copy of the first edition, February issue, Showa 61 (1986), pp. 101-107, edited by The Society of Powder Technology, Japan, published from THE NIKKAN KOGYO SHIMBUN, LTD.)
Nonpatent Document 1 describes that electrically conductive powder for high-density packing is prepared by mixing spherical particles of various sizes. Nonpatent Document 1 discloses that the spherical particles are regularly arranged and small spherical particles are packed in spacing, whereby a packing density of at least 80% is theoretically accomplished. However, this is not always the case for the following reasons. Commercially available spherical silver powder contains partially agglomerated particles. Silver powder having particle diameters of 3 to 20 μm achieves a relative packing density of about 60%, and silver powder with particle diameters around 1 μm has a relative packing density as high as only 50%. Even if these powders are mixed together, the relative packing density is only about 60%.
Electrical contact between circuits of a multilayer circuit board is established by filling through-holes with an electrically conductive packing paste. In order to obtain high electric conductivities, it is necessary that as much as possible of the electrically conductive paste is filled in the through-holes and the electrically conductive powder fills the holes without spacing. Because of this requirement, the conventional electrically conductive packing pastes used in such field are required to contain a large amount of electrically conductive powder. However, increasing a contact of the electrically conductive powder results in high viscosity of the electrically conductive paste, and the paste will not geed-fill through-holes, well. The viscosity is reduced by increasing the binder content in the electrically conductive paste, and such paste will easily fill through-holes. However, electric conductivities are deteriorated at the same time.
When a conventional electrically conductive paste is used as a thermally conductive adhesive, thermal conductivity of the adhesive is deteriorated even when the electrically conductive powder consists of spherical particles, irrespective of the packing density of the electrically conductive powder.
Because electrically conductive spherical particles contact each other or contact surfaces at only points, the contact efficiency is bad. Even if substantially flaky particles are used in order to improve the contact efficiency, the paste viscosity will also increase, and the paste does not fill via-holes in a circuit board sufficiently. Moreover, when the paste is filled in the via-holes, the electrically conductive flaky particles tend to be oriented by the paste's viscous behavior so that the major surface of the flaky particles is perpendicular to the Z axis (conductive direction) of the via-holes. Consequently, electrical and thermal conductivities in the Z axis direction are much lower than expected.
The electrically conductive spherical particles have another problem. When a layer of the particles is pushed down with a press or the like, the particles are isotropically pressed and easily slide on one another. As a result, a packing volume of them deteriorates, and the electrically conductive particles are difficult to press strongly each other.
The conventional electrically conductive pastes containing electrically conductive powder are used as electrically conductive adhesives. If the electrically conductive adhesive has low thixotropic properties, the paste will be stringy and easily blot the electrically conductive adhesive may attach to undesired next portions.
The use of fine electrically conductive particles and fine electrically conductive flaky particles in combination will increase the thixotropic properties of the electrically conductive adhesive. However, because the fine particles are agglomerated, an electrically conductive adhesive paste containing such particles has high viscosity and does not fill holes enough. In a paste prepared using an electrically conductive mixed powder with a low relative packing density, when increasing the electrically conductive mixed powder, it is difficult to produce a paste because the obtained mixture has very high viscosity.